1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile, a multifunctional apparatus, and the like.
2. Description of Related Art
Among the image forming apparatuses using toner, such as a copier, a printer, a facsimile, a multifunctional apparatus, and the like, some have been provided with a photoconductive drum and a developing roller arranged opposite the photoconductive drum with a gap in between. And the so-called developing bias voltage obtained by superimposing a DC component on an AC component is applied to the developing roller. As a result, electrically charged toner particles are transferred from the developing roller to the photoconductive drum, and thereby an electrostatic latent image is developed.
So that the density of an image to be formed is secured by sufficiently supplying toner particles to the photoconductive drum, with the aim of increasing developing efficiency, the alternating (AC) voltage applied to the developing roller has simply to make its peak-to-peak voltage high. Making it too high, however, leads to electrical discharge occurring in the gap between the photoconductive drum and the developing roller. If electrical discharge occurs there, owing to a change in potential on the photoconductive drum surface, an electrostatic latent image will be disturbed, with the result that the quality of a resulting image is degraded. Moreover, a large current will possibly be rushed into the photoconductive drum, making it damaged. Thus, even in a case where the peak-to-peak voltage of the AC voltage is made high, such the voltage leading to electrical discharge should not be applied to the developing roller during the image forming operation.
Thus, so that the developing efficiency is increased with no problem arising from electrical discharge, the alternating (AC) voltage that does not lead to electrical discharge between the photoconductive drum and the developing roller when engaging in the image forming operation, and that is as high as possible is applied to the developing roller. For example, the magnitude of the AC voltage applied to the developing roller is altered to detect the occurrence or non-occurrence of electrical discharge and to thereby find out a peak-to-peak voltage at which the occurrence of electrical discharge is started. Then a potential difference between the developing roller and the photoconductive drum at a time when the electrical discharge has occurred is grasped. After that, setting is done to specify the AC voltage applied to the developing roller so that the image forming operation is performed with a potential difference between the developing roller and the photoconductive drum slightly lower than the potential difference thus grasped.
For example, JP-3815356 discloses a developing apparatus including: an image carrier; and a toner carrier arranged opposite the image carrier with a predetermined interval in between inside a developing region, wherein a developing bias voltage with a direct (DC) voltage superimposed on an alternating (AC) voltage is applied between the toner carrier and the image carrier, toner is supplied to the image carrier, and an electrostatic latent image is developed; the developing apparatus further includes: leak generation means changing a leak detection voltage that is applied between the image carrier and the toner carrier; and a leak detection means detecting a leak, wherein when a maximum potential difference ΔVmax between the leak detection voltage and a potential at a surface of the image carrier is gradually increased, and if a current passing through the image carrier and the toner carrier is successively increased, the leak detection means considers it as the leak (e.g., see JP-3815356, specifically claim 1 and others).
As an example, FIG. 16 shows, by way of example, a relationship of the potential difference between the photoconductive drum and the developing roller versus a discharge current passing through the photoconductive drum and the developing roller. FIG. 16 illustrates a case in which a photoconductive drum having a photoconductive layer formed of amorphous silicon and positively charged is employed. In the example shown in FIG. 16, when the potential of the developing roller is lower than that of the photoconductive drum (in a negative direction), if the potential difference between the developing roller and the photoconductive drum exceeds a certain value, a discharge current is dramatically increased. On the other hand, when the potential of the developing roller is higher than that of the photoconductive drum (in a positive direction), even if the potential difference exceeds that certain value, an increase in the discharge current is moderate compared with that when the potential is in the negative direction. This feature can be observed with the photoconductive layer formed of any other material.
In the developing apparatus disclosed by JP-3815356, the leak detection voltage is altered, and thus, the electrical discharge may take place in the negative direction, possibly leading to the large amount of discharge current made to pass. Additionally, an increase in current is checked by gradually increasing the maximum potential difference ΔVmax between the leak detection voltage and a surface potential of the image carrier. Thus, there is a strong possibility that an accordingly large discharge current forms an ultra-small hole called “drum pinhole” in the photoconductive drum. That is, the photoconductive drum is highly likely to be damaged. If such a drum pinhole is formed, it is impossible to carry electrical charges and hence toner particles there. This adversely affects the quality of an image to be formed in the image forming operation.